1. Field of the Invention
The present invention relates to frequency demodulation for demodulating a frequency-modulated signal, and more particularly, to a frequency demodulation apparatus and method for demodulating the signal using an arcsin approximation.
2. Description of the Related Art
A conventional apparatus for demodulating a frequency-modulated analog signal is disclosed in U.S. Pat. Nos. 5,194,938 and 4,232,268, incorporated herein by reference. The disclosed conventional frequency demodulation apparatus is large and complicated due to the need for an accurate phase locked loop (PLL). In addition, the first conventional frequency demodulation apparatus performs analog frequency demodulation which is greatly affected by process parameters, resulting in increased circuit size. Furthermore, in the case where the conventional frequency demodulation apparatus is employed to demodulate a SECAM (SEquential Couleur Avec Memoire or SEquential Color with Memory) color difference signal, frequencies of sub-carriers for the lines corresponding to first and second color difference signals Db and Dr are different. For this reason, a PLL for each line must be provided.
A second conventional frequency demodulation apparatus has been disclosed in an article entitled xe2x80x9cAn arctangent type wideband PM/FM demodulator with improved performancexe2x80x9d, IEEE TRANSACTION ON CONSUMER ELECTRONICS, Vol. 38, No. 1, pp 5-9, in February 1992, incorporated herein by reference. However, the arctangent approximation used in the second conventional frequency demodulation apparatus results in a large amount of error. Furthermore, the second conventional frequency demodulation apparatus requires a look-up table for the arctangent function in order to attain desired performance, the hardware complexity and manufacturing costs increase.
A third conventional frequency demodulation apparatus has been disclosed in an article entitled xe2x80x9cA novel FM demodulation schemexe2x80x9d, IEEE TRANSACTION ON CONSUMER ELECTRONICS, Vol. 41, No. 4, pp 1103-1107 in November 1995. This technique requires an additional look up table for calculating a square root function, and, as resolution increases, so too must the size of the look-up table, thus once again, increasing hardware complexity and cost.
To address the above limitations, it is a first object of the present invention to provide a frequency demodulation apparatus for performing frequency demodulation using an arcsin approximation.
It is a second object of the present invention to provide a frequency demodulation method performed in the frequency demodulation apparatus.
It is a third object of the present invention to provide a frequency demodulation apparatus for SECAM (SEquential Couleur Avec Memoire or SEquential Color with Memory) chroma demodulation, which demodulates the frequency of color difference signals transmitted according to the SECAM protocol, using an arcsin approximation.
It is a fourth object of the present invention to provide a frequency demodulation method for SECAM chroma demodulation, performed by the frequency demodulation apparatus for the SECAM chroma demodulation.
Accordingly, to achieve the above first object, there is provided a frequency demodulation apparatus comprising: an analog-to-digital converter (ADC) for sampling a frequency-modulated analog signal to convert the frequency-modulated analog signal into a digital signal, and outputting the sampled values (S0xcx9cSn) of the digital signal; a first multiplier for multiplying a sinusoidal first and a second oscillation signals having a phase difference of 90xc2x0, by the sampled value Si (0xe2x89xa6ixe2x89xa6n) respectively, and outputting the products as the signals Ii and Qi; a low pass filter for low-pass-filtering the signals Ii and Qi and outputting the low-pass-filtered signals Iixe2x80x2 and Qixe2x80x2; a first de-emphasizer for de-emphasizing the high-frequency components of the signals Iixe2x80x2 and Qixe2x80x2 and outputting the de-emphasized signals Iixe2x80x3 and Qixe2x80x3; a frequency differentiator for delaying the signals Iixe2x80x3 and Qixe2x80x3, multiplying and then subtracting the delayed signals, and outputting the subtraction result as a frequency-demodulated digital signal Z; a gain corrector for correcting the gain of the signal Z; and an oscillator for outputting the first and second oscillation signals.
To achieve the second object, there is provided a frequency demodulation method comprising the steps of: (a) sampling a frequency-modulated analog signal to calculate sampled values S0xcx9cSn; (b) multiplying first and second oscillation signals that are sinusoidal with a phase difference of 90xc2x0 by the sampled value Si (0xe2x89xa6ixe2x89xa6n) respectively and obtaining the products Ii and Qi; (c) low-pass-filtering the signals Ii and Qi to obtain the low-pass-filtered signals Iixe2x80x2 and Qixe2x80x2; (d) de-emphasizing the high-frequency components of the signals Iixe2x80x2 and Qixe2x80x2 to obtain the de-emphasized signals Iixe2x80x3 and Qixe2x80x3; (e) delaying the signals Iixe2x80x3 and Qixe2x80x3, multiplying and then subtracting the delayed signals to obtain a frequency-demodulated digital signal Z; and (f) correcting the gain of the frequency-demodulated digital signal Z.
To achieve the third object, i.e., for the SECAM chroma demodulation, the frequency demodulation apparatus further comprises: a first selector for selectively outputting the signals Iixe2x80x3 and Qixe2x80x3 or the signals Iixe2x80x2 and Qixe2x80x2 in response to a selection signal; a gain adjustor for determining whether the amplitude of the signal Iixe2x80x2 maintains a predetermined value, adjusting the gain of the signal Iixe2x80x2 in response to the determination result, and outputting the gain-adjusted signal to the first multiplier; a line checking unit for checking whether the current line is an even line or odd line using the signal Z, the selection signal and the delayed signals, and outputting the checking result; a second de-emphasizer for de-emphasizing the low-frequency component of the signal output from the gain corrector, and outputting the de-emphasized result; and a color difference signal reproducer for differentiating and outputting the signals for the previous line among the signals output from the second de-emphasizer as a frequency-demodulated first color difference signal and for differentiating and outputting the signals for the current line among the signals output from the second de-emphasizer as a frequency-demodulated second color difference signal, in response to the line checking result of the line checking unit, wherein the analog signal includes the frequency-modulated first or second color difference signal, the color difference signal transmitted being loaded on the odd or even line, the frequency differentiator calculates the signal Z using the result selected by the first selector, (instead of using the signals Iixe2x80x3 and Qixe2x80x3), the first multiplier multiplies the signals Ii and Qi by the gain adjusted by the gain adjustor and outputs the products to the low pass filter, the gain corrector corrects the gain and offset of the signal Z in response to the check result of the line checking unit and outputs the corrected result to the second de-emphasizer, and the selection signal is generated during the interval in which a color burst signal exists in the analog signal.
To achieve the fourth object, there is provided a frequency demodulation method for the SECAM chroma demodulation, comprising the steps of: (a) sampling an analog composite video baseband signal with a frequency-modulated first or second color difference signal to obtain sampled values S0xcx9cSn; (b) multiplying third and fourth oscillation signals that are sinusoidal with a phase difference of 90xc2x0 by the sampled value Si (0xe2x89xa6ixe2x89xa6n) respectively and obtaining the products Ii and Qi; (c) low-pass-filtering the signals Ii and Qi to obtain the low-pass-filtered signals Iixe2x80x2 and Qixe2x80x2; (d) delaying the signals Iixe2x80x2 and Qixe2x80x2, multiplying and then subtracting the delayed signals to obtain a signal Z; (e) determining whether the current line is an even line or odd line using the signal Z, the selection signal and the delayed signals; (f) determining the signals Z for the previous line as a frequency-demodulated first color difference signal if the current line is an even line; and (g) determining the signals Z for the current line as a frequency-demodulated second color difference signal if the current line is an odd line, wherein the frequency-modulated first and second color difference signals are loaded on the odd or even line and transmitted.